In general, an optical scanning device used for an image-forming device such as a laser printer, a digital copier, and a multi-functional printer is designed so that a light flux from a laser light source is deflected by a deflection optical system such as a polygon mirror, and is formed into an image as a light spot on a scanning surface by a scanning and image-forming optical system.
As the laser light source, a semiconductor laser and the like is commonly used. A divergent light emitted from the laser light source is converted into an almost parallel beam by a collimator lens, and the beam is subjected to a restriction of the shape by an aperture. The beam in which the shape was restricted is deflected by the deflection optical system such as a polygon mirror rotating at a fixed angular speed to enter the scanning and image-forming optical system. The scanning and image-forming optical system has a fθ characteristic, which scans a scanning surfaces arranged at predetermined spaces at a equidistant speed with a beam deflected at a fixed angular speed, and is required that curvature of field is successfully corrected to form a minute light spot over the whole scanning area.
In case where the polygon mirror is used in the deflection optical system, since the polygon mirror has an error in a mirror surface processing and a vibration of rotation axis, most of image-forming lens systems are provided with a face-tangle-error correction function to correct a shift in the scanning location in a sub-scanning direction (being a perpendicular direction to a main-scanning direction). Therefore, the image-forming lens system is designed to be an anamorphic lens system having different image forming characteristics between a main-scanning direction and a sub-scanning direction.
Heretofore, the above scanning and image-forming optical system has been manufactured out of glass materials. However, since a glass lens processing is difficult resulting in a high cost, it has been desired in recent years to manufacture the above scanning and image-forming optical system out of plastic materials in which the cost is low and an aberration can be compensated with a free form.
Further, heretofore, there has been provided an infrared laser (with the wavelength of about 780 nm), or a red laser (with the wavelength of about 650 nm), as a semiconductor laser used for a light source has, in general. In recent years, due to demand for higher resolution, a light scanning device has been developing using a light source of a short wavelength of 500 nm or less by which a minute spot shape can be obtained. Further, in recent years, due to demand for making the device smaller, it has also been desired to make the polygon mirror or the plastic lens smaller by making a beam size of a light source smaller by increasing the F number of the scanning and image-forming optical system using a light source with a short wavelength.
Most of the optical materials used for a plastic lens tend to decrease the transmittance due to internal absorption of the materials as the wavelength becomes shorter. In particular, when a light source with a short wavelength of 500 nm or less is used, the decrease in transmittance due to internal absorption becomes larger. Further, it was found that there is a problem that the transmittance further decreases due to cloudiness of the plastic lens when short-wavelength light is irradiated to the plastic lens for a long time.
To cope with the above problem, as a light scanning device using a light source of 500 nm or less, there has been proposed a light scanning device in which at least one lens in the scanning and image-forming optical system is a plastic lens (refer to Patent Literature 1).
In addition, there has been proposed a light scanning device in which, using a light source of 450 nm or less, an optical system between a light source and a deflection optical system is an optical element made of glass, and an scanning and image-forming optical system is an optical system made of plastic (refer to Patent Literature 2). This light scanning device is also trying to lower the cost in such a way that the optical element, in which a light flux continually transmits, is made of a glass optical element, and, on the other hand, the optical element of the scanning and image-forming optical system, in which a light flux does not always passes through, is made of a plastic optical element.    Patent Literature 1: Japanese Patent Document No. 3478819    Patent Literature 2: Japanese Patent Application Publication No. 2006-313268